Proteins are the most abundant organic compounds to be found in cells and comprise over 50% of their total dry weight. They are present in all types of cells and in proteins perform many functions. They build many structures of the cell. All enzymes are proteins and in this way they control the whole metabolism of the cell. As hormones, proteins regulate metabolic processes. Some proteins (e.g. hemoglobin) work as carriers and transport specific substances such as oxygen. Lipids, metal ions, etc. some proteins called antibodies, defend the body after an injury. Movement of organs and organisms, and movement of chromosomes during anaphase of cell division, are caused by proteins.
Proteins are polymers of amino acids, the compounds containing carbon, nitrogen, oxygen and hydrogen. The number of amino acids varies from a few to 3000 or even more in different proteins.
Amino acids: about 170 types of amino acids have been found to occur in cells and tissues. Of these, about 25 are constituents of proteins. Most of the proteins are however, made of 20 types of amino acids.
All the amino acids have an amino group (-NH2) and a carboxyl group (-COOH) attached to the same carbon atom, also known as alpha carbon. They have the general formula as:
R may be a hydrogen atom as in glycine, or CH3 as in alanine, or any other group. So amino acids mainly differ due to the type of nature of R group.
Amino acids are linked together to form polypeptides proteins. The amino group of one amino acid may react with the carboxyl group of carboxyl group of one amino acid and the hydrogen of amino group of another amino acid release H2O and C – N link to form a bond called peptide bond. The resultant compound glycylalanine, has two amino acid subunits ant is a dipeptide. A dipeptide has an amino group at one end and a carboxyl group at the other end of the molecule. So both reactive parts are again available for further peptide bonds to produce tripeptides, tetrapeptides, and pentapeptides etc, leading to polypeptide chains.
Fig. 2.10:peptide linkage – formation of peptide bond